Electrical generators may be used in many different fields. When a generator is e.g. used in a wind turbine, one of the more important economic parameters, with respect to the dimensioning of the wind turbine, is the size of the housing. It is therefore of great importance to be able to minimize the diameter of the wind turbine. In order to minimize the housing one has to minimize the gearbox/gear wheel connecting the wing and the generator. This can be achieved by providing a generator which has a relatively large effect per revolution.
One way to achieve this is to have a generator with as small a radial extent as possible, since the generator occupies a relative large amount of space in the housing of the wind turbine.
Another aspect to be considered when implementing generators in wind turbines is that the generator has to be effective both at a low and a high number of revolutions.
An electrical machine based on a conventional radial flux generator, see FIG. 1, is most frequently used. A main problem with generators of this kind in certain situations is that the diameter for a given power output is relatively large, because of the radially built stator construction. A further disadvantage is that the stator surrounds/encircles the rotor, thereby adding to the diameter of the generator.
Another disadvantage is the relative low induction in the air gap caused by the individual arrangement of the material between the recess 7 and the recess 2 themselves, since only the material 7 carries the flux and only covers about 50% of the free space toward the gap.
There are many generators of similar kind, which are optimized in one way or another, but they all have a radial flux and thus involve the same problem, i.e. a relatively larger diameter, like the one described above.
A motor or generator having an axial flux, see FIG. 2, is proposed in WO 00/48247, which is hereby included by reference. Here, a motor or generator is provided having a magnetic flux path through one or more pole cores 15 surrounded by current windings 16 and extending in the axial direction. This allows a high density of the magnetic flux to be passed through the pole cores 15, resulting in a low consumption of material for the pole cores when compared to machines, where for example a large stator diameter may be needed in order to conduct a high magnetic flux. By having the pole cores 15 arranged parallel to the axis of the rotor 10, the overall diameter of the motor or generator may be reduced, thus providing a solution to some of the above-mentioned problems.
For the motor or generator described in WO 00/48247 the number of pole cores or pole legs 15 arranged in the stator equals the number of magnets arranged in the rotor, and according to the embodiment illustrated in FIG. 2, the motor or generator comprises one rotor 10 and one stator. The rotor 10 has a number of pole shoes 13, disposed between magnets 12. The stator comprises a number of pole cores or pole legs 15, where the number of pole legs 15 equals the number of magnets 12, which again equals the number of pole shoes 13. There are two adjacent local magnetic circuits for each given pole core 15. Two of these are schematically illustrated by the first and second loops 18a, 18b. It is seen that when the pole shoes 13 are facing the pole legs 15, a magnetic flux path 18a includes a first pole leg, a first pole shoe, a magnet, a second pole shoe, and a second pole leg.
In FIG. 2, the density of the magnetic flux in the flux path 18a or 18b is relatively high, leading to a high resulting axial magnetic force between the rotor 10 and pole legs 15 of the stator. When the stator 10 is rotated so that each magnet 12 is now facing a pole leg 15, a third magnetic flux path will include only one pole leg 15, a first pole shoe, a magnet, and a second pole shoe (this situation is illustrated in FIG. 3). The magnetic flux density of the third flux path is lower than for the first and the second flux paths 18a, 18b, leading to a lower resulting axial magnetic force between the rotor 10 and the pole legs 15, when compared to the rotor position illustrated in FIG. 2. So, when the rotor 10 is rotated during use, the resulting axial force between the rotor 10 and the pole legs 15 of the stator will vary between a relatively high and a relatively low force. Such a high, varying axial force may result in several drawbacks including a high wear on the rotor 10 and its axial connection.
According to another embodiment of the motor or generator described in WO 00/48247 having an axial flux and illustrated in FIG. 3, the motor or generator comprises one rotor 301 and two stators 302, 303 arranged on opposite sides of the rotor 301. The rotor 301 has pole shoes 304, 305 and a magnet 306, 307, 308 between each two succeeding pole shoes. The pole shoes 304, 305 are crossing the rotor 301, whereby pole shoes are provided on each side of the rotor 301. The first stator 302 has pole cores or pole legs 309, 310 facing the poles shoes 304, 304 of the rotor 301, while the second stator 303 has pole cores or pole legs 311, 312, 313 facing the magnets 306, 307, 308 of the rotor 301. Here, the pole legs 309, 310 of the first stator 302 is displaced compared to the position of the pole legs 311, 312, 313 of the second stator 303.
For the generator of FIG. 3, a first magnetic flux path 314 of the rotor 301 and the first stator 302 includes the pole legs 310, 309, the pole shoe 304, the magnet 307, and the pole shoe 305. However, a second magnetic flux path 315 exists corresponding to the flux path 314. This second magnetic flux path 315 includes only one pole leg 312, the pole shoe 304, the magnet 307, and the pole shoe 305. It should be understood that as the number of pole legs in the stators 302, 303 equals the number of magnets in the rotor 301, similar corresponding magnetic flux paths exist for the remaining stator pole legs and rotor pole shoes and magnets.
Here, the density of the magnetic flux in flux path 314 is much higher than the density of the magnetic flux in flux path 315. Thus, the resulting axial magnetic force between the rotor 301 and the first stator 302 is much higher than the resulting and oppositely directed axial magnetic force between the rotor 301 and the second stator 303. However, when the rotor 301 is rotated so that the pole shoes 304, 305 are now facing pole legs 312, 313, respectively of the second stator 302, while the magnet 307 is facing pole leg 310, the magnitudes of the oppositely directed axial magnetic forces between the rotor 301 and the two stators 302, 303 changes, so that the force between the rotor 301 and first stator 302 is lower than the force between the rotor 301 and the second stator 302.
So, when the rotor 301 is rotating during use, the maximum axial force on the rotor 301 is high, but changes in direction during the rotation. Such a high, varying axial force may result in several drawbacks including a high wear on the rotor 301 and its axial connection.
Thus, there is a need for a design of a motor or generator having an axial flux, but having only a relatively small variation in the varying axial force on the rotor to thereby reduce the wear of the rotating parts.